A key dilemma facing organ and tissue grafting is the shortage of donor tissues for transplantation. This problem has led to the consideration of xenogeneic tissues as an alternative supply of donor material. The aim of this proposal is to understand the nature of T cell-dependent immunity/tolerance to xenogeneic pancreatic islets as a model of cellular xenotransplantation. Our general working hypothesis has been that islet allograft rejection is dominated by donor APC-dependent 'direct' recognition while xenograft rejection is dominated by host APC dependent 'indirect' recognition. Based on this supposition, we had proposed that the nature of tolerance to islet allografts and xenografts might be quite distinct. However, many studies increasingly suggest that host MHC class II-restricted (indirect) antigen presentation may be of primary general importance in peripheral tolerance. Thus, xenograft tolerance and allograft tolerance may not be as distinct as we once imagined. Consistent with this view, we have found that robust therapies for inducing islet allograft tolerance are nearly as efficacious for inducing long-term rat and porcine islet xenograft survival in mice. In particular, we have found that combined therapy with anti-LFA-1 (CDlla) plus anti-CD154 monoclonal antibodies can lead to consistent long-term allograft, and rat or porcine xenograft acceptance in high-responder mouse strains. A key goal of this proposal will be to determine if putative properties of peripheral allograft tolerance also apply to 'rules' governing xenograft 'tolerance' in vivo. To this end, this proposal has the following specific aims: (1) Determine whether long-term xenograft acceptance is associated with regulatory tolerance in vivo: This aim will test the simple hypothesis that xenograft tolerance induced in adult animals results is due dominant, regulatory tolerance. (2) Determine the cellular requirements for long-term xenograft acceptance/tolerance. This Aim will test the hypothesis that long term xenograft acceptance requires the active participation of lymphoid subpopulations including CD4 T cells, CDl-restricted 'invariant' (TcR Jalpha281+) NKT cells, and B cells, and (3) Determine whether IFNgamma is necessary for xenograft acceptance/tolerance. This Aim will test the hypothesis that, unlike several models of allograft tolerance, xenograft tolerance will be IFNgamma-independent in vivo.